1. Field of the Invention
The present invention relates to pressure relief valves, and in particular, pressure relief valves permitting bi-directional flow responsive to changes in pressure differential.
2. Prior Art
One-way check valves of the so called "duckbill" type generally incorporate a hollow cylindrical body terminating in a duckbill which permits flow in one direction but checks flow in the opposite direction.
In some environments it is desirable to not only permit flow in one direction and check flow in the opposite direction, but to also provide for reverse flow or pressure relief when the flow pressure in the opposite direction exceeds a certain value.
U.S. Pat. No. 3,155,110 discloses various embodiments of check-relief valves including one that incorporates a coil spring and another which utilizes collapsible lugs or feet. In both versions, reverse flow is provided by an axial shifting of the entire valve body.
U.S. Pat. No. 3,159,176 discloses a check-relief duckbill valve in which reverse flow is accomplished, not by axial movement of the valve, but by the provision of a flexible umbrella portion which lifts off its seat at reverse flow pressures in excess of some value to permit reverse flow around the outside of the body of the valve.
Obviously the use of a separate coil spring makes the valve assembly more complicated and expensive, while it is often impractical or even impossible to utilize a valve which relies upon axial movement of the entire valve or flow around the outside of the valve body to provide reverse flow.
U.S. Pat. No. 4,181,145 discloses a two-way check valve comprising a valve body having an opening covered by a flexible wall such that it defines two slits. The walls of the opening are sloped such that a forward flow pressure across the opening causes forward flow through a first slit while the other remains closed, and reverse flow pressure causes flow in a reverse direction through the second slit while the first remains closed. Thus, this two-way check valve actually consists of two one-way valves oriented to permit flow in opposite directions.